#include <string.h>
#include <stdlib.h>
#include <stddef.h>

struct big {
    unsigned long long ull_a;
    unsigned long long ull_b;
    unsigned long long ull_c;
    int si_e;
    int si_f;
};

void func(void){
    size_t skip = offsetof(struct big,ull_b);
    struct big *s = (struct big *)malloc(sizeof(struct big));
    if (s == NULL){
        /* Handle malloc error */
    }
    memset(s + skip,0,sizeof(struct big) - skip);
    /* ... */
    free(s);
    s = NULL;
}